Sanchez sure butchered a lot of fighters. looks at Laport's face. look at Gomez's face. look at Lopez's face
A large part of having a "great chin" has to do with having great balance, which is such an overlooked but important part of boxing. An off balance fighter is a lot easier to knock down than a well balanced fighter. Sanchez had great balance, comparable to other fighters with great balance such as Hagler or SRR.
It was the first Sanchez fight I watched. I think I last watched it over 10 years ago. I remember on the version I saw that Angelo Dundee was co-commentating and like most people at that point hadn't seen Sanchez fight before. About halfway through there's a realisation from the commentary team that they're watching someone who's a bit special and Dundee exclaims something like 'this Sanchez is a helluva fighter!' Sometimes a fighter's greatness is just obvious to see and it certainly was the case with Sanchez.